This question doesn't pertain to a particular Boost library, so I apologize if it's too off-topic for the list. According to http://www.boost.org/more/generic_programming.html , the Associated Types of a Concept are either nested in the class definition, or accessed through a traits class. With iterators, traits were used because you can't nest typedefs within pointers, which are valid iterators in many cases. With containers (such as AssociativeContainerConcept), the Concepts require nested typedefs. My questions are - when creating your own library, when do you choose nested typedefs over a traits class, and what effects does it have on a library user, both from a usability and extensibility point of view. What questions do you ask yourself to come to a decision? Thanks, --Michael Fawcett